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#41
07-28-2006, 10:16 PM
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Re: Random question out of nowhere
[ QUOTE ]
You are stupid if you've been wondering about this for any length of time. F = MA [/ QUOTE ] Pretty sure F=ma is not the way to go about this. The mass of the person falling is constant, the acceleration due to gravity is constant, so the Force is the same regardless of whether the person falls out of a 2nd story or 10th story window. But logically, falling out of a 10th window would cause more damage. So, there has to be another way to solve the problem, but I don't remember how (momentum?). 
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#43
07-29-2006, 12:35 AM
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Re: Random question out of nowhere
[ QUOTE ]
[ QUOTE ] You are stupid if you've been wondering about this for any length of time. F = MA [/ QUOTE ] Pretty sure F=ma is not the way to go about this. The mass of the person falling is constant, the acceleration due to gravity is constant, so the Force is the same regardless of whether the person falls out of a 2nd story or 10th story window. But logically, falling out of a 10th window would cause more damage. So, there has to be another way to solve the problem, but I don't remember how (momentum?). [/ QUOTE ] momentum, yes. MA just tells you how much force you need to use to stop the guy from accelerating downwards. the problem is that he already has a ton of momentum (MV). 5 floors sounds like about 25 meters, which i think is a bit over 20m/s by the time he hits the ground.
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#45
07-29-2006, 10:11 PM
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Re: Random question out of nowhere
its 22 m/s, using kinematic equation of vf^2=vi^2+2ad. Vi=0 and Vf is what your solving for. Nice guess xorbie? This is ignoring air friction. The formula used for impulse is mv=Ft. This equation assumes the same force is constantly applied when deaccelerating the body. This probably isn't true, and is the whole reason for integrating the force over time....
The problem with your question is its hard to determine the impulse time (what diablo hinted at earlier). The force is what should be solved for. Then you'd have to know what kind of stress a persons bones/organs can withstand. Using stress=F/a. It'd be much easier to figure out if the guy fell on a car. Comparing the maximum stress a bone can withstand with relation to the force exerted through the impulse would give an answer. From the stress equation, it matters how big of an area supports his fall. It also matters over how long of a time period he's deaccelerating. This makes the guy falling on a car much different than being caught by another guy. The guy falling on the car has a bigger contact area to land on. Also steel stresses are known quite well. There was confusion earlier over F=ma and its application to this. That equation just describes the acceleration of the body. To describe an impact, impulse is what your really wondering about. One final nitty comment. Some people were trying to plug pound figures into mass; pounds is a measure of force. The american unit for mass is slugs. Another problem in calculating when a guy catches him is moment force. If he catches him with his arms, there's going to be a moment created. Moment=forceXdistance (vector equation using cross products). A moment is the reason its easier to open a door using the handle than pushing right by the hinges. This question is extremely complex. Probably the reason they have crash test dummies to test cars....
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